1. Field of the Invention
The present invention relates generally to computer systems equipped with video capabilities and a Universal Serial Bus (USB), and specifically to combining video data with USB data transfers over an extended USB.
2. Description of the Related Art
The Universal Serial Bus (USB) is a serial bus standard that provides a method of coupling peripheral devices to a computer system. USB was developed by Intel Corporation as a general purpose port, with the intention of eliminating jumpers, IRQ settings, DMA channels, and I/O addresses, USB supports data exchange between a host computer and a wide range of simultaneously accessible devices which share USB bandwidth through a token scheduled protocol. The bus allows peripherals to be attached, configured, used, and detached while the host is in operation. The USB allows many (e.g., up to 127) devices to be daisy-chained with a single standard connector. USB supports devices that transfer data from 1.5 Mbps to 12 Mbps, and is expected to support transfer rates up to 480 Mbps under the USB 2.0 Protocol Specification. By continually polling the bus for devices, users may xe2x80x9chot-plugxe2x80x9d peripherals into the system and use them without rebooting.
The USB technology may greatly simplify the complex cabling that typically spills out from the back of personal computers. USB peripherals may include keyboard, mouse, phone/answering machine, printer, scanner, fax/modem, ISDN, tablet, game controller, light pen, digital audio, and any other USB compliant device. The USB protocol assumes a short bi-directional connection between the local and remote ends of the USB network. The consequently short latencies for packet transmission allow the USB to be transaction oriented (e.g. token, data, and handshake are all completed before the next transaction begins) with very little performance loss.
The USB cable is a four wire cable, and the maximum cable length is about 5 meters. There are typically two connector types, and no cross-over cables and adapters are needed. The maximum USB cable length of about 5 meters results from the fact that the USB Controller considers any transmission return time greater than a characteristic threshold value to be an error. USB cable lengths greater than about 5 meters may generate longer return times than the specified threshold value, and thus 5 meters is the maximum USB cable length. This 5 meter constraint may severely restrict the manner in which USB is used, especially given the fact that peripherals may be chained together sequentially. For example, in some situations it may be desirable to operate a USB and associated USB peripherals at a remote location from the associated host computer.
FIG. 1: A Host Computer With USB Peripherals
FIG. 1 illustrates a USB system. As FIG. 1 shows, a host computer system 108 may be coupled to various USB compliant peripherals, such as a keyboard 110A, and a mouse 110B through a Universal Serial Bus (USB) 220. A display device or monitor may also be connected to the computer system 108 through a monitor/video cable.
FIG. 2: A Block Diagram Of a Host Computer With USB Peripherals
FIG. 2 is a block diagram of a host coupled to a variety of peripheral devices through a USB. As FIG. 2 shows, the host computer system 108 may be coupled to USB compliant peripherals, including keyboard 110A, and mouse 110B through Universal Serial Bus (USB) 220. Host computer 108 may include a USB Controller 230 for coupling to USB communication medium 220. Host computer 108 may be operable to send and receive data to and from the USB peripherals shown through USB Controller 230. Host computer 108 may include USB driver software 240 which interfaces to the USB Controller 230 and facilitates communication with the USB peripheral devices. As mentioned above, there is a requirement that the total USB cable length not exceed 5 meters.
FIG. 3A: USB System Software Architecture
FIG. 3A is a block diagram of the software architecture of a USB system. As FIG. 3A shows, the top layer of the software architecture is application software 302. The application software 302 may be any software program which may be operable to provide an interface for control of or communication with a USB peripheral device. A USB driver program 240 may be below the application software 302. The next software layer may be OHCI driver software 306, which interfaces with the relevant hardware; i.e., the USB Controller hardware 230. The USB Controller hardware 230 communicates through USB bus 102 to various USB peripherals 110.
FIG. 3B: USB System Software/Hardware Architecture
As FIG. 3B shows, system software 310 may include software modules that effect USB operation including client driver software 311 (which may be used by application software 302), USB driver 240, and a Universal Host Controller Driver (HCD) 306. As FIG. 3B also indicates, a hardware implementation of the system is shown in hardware 230, including Universal Host Controller (HC) 230 and USB device 110, which may be coupled by USB 102. The timing issues which results in the cable length problem exist in the host controller 230, which relies on timely acknowledgements from USB devices 110 as described in the USB specification. The host controller 312 is specified in the USB standard as having a temporal window of valid reception after each transmission between any two USB devices (controller, hubs, user devices, etc.). This period of time is 70 nanoseconds, 30 nanoseconds of which may be committed to electronic processing time in the USB Controller hardware and the other 40 nanoseconds represents the maximum time-of-flight of the data through the connecting cable. This 40 nanosecond time-of-flight issue influences the cable length limit. This timing issue is endemic to the USB process and cannot be altered. Such timing issues are described in detail below with reference to FIG. 5.
FIG. 4: USB Data Delivery Packets
The USB is a polled bus, which means the host controller initiates all data transfers. Most bus transactions involve the transmission of up to three packets. FIG. 4 is a block diagram of a typical bus transaction. Each transaction begins when the host controller, on a scheduled basis, sends a USB packet 402 describing the type and direction of transaction 410, the USB device address 412, and endpoint number 414. This packet may be referred to as the xe2x80x9ctoken packet.xe2x80x9d The USB device that is addressed selects itself by decoding the appropriate address fields.
In a given transaction, data may be transferred either from the host to a device or from a device to the host. The direction of data transfer may be specified in the token packet 402. As FIG. 4 shows, the source of the transaction then sends a data packet 404 containing the data to be transferred 416 or indicates it has no data to transfer. The destination, in general, responds with a handshake packet 408 indicating whether the transfer was successful 418.
Some bus transactions between host controllers and hubs involve the transmission of four packets. These types of transactions may be used to manage the data transfers between the host and full-/low- speed devices. The USB data transfer model between a source or destination on the host and an endpoint on a device may be referred to as a pipe. There are generally two types of pipes: stream and message. Stream data has no USB-defined structure, while message data does. Additionally, pipes have associations of data bandwidth, transfer service type, and endpoint characteristics like directionality and buffer sizes. Most pipes come into existence when a USB device is configured. One message pipe, the Default Control Pipe, always exists once a device is powered, in order to provide access to the device""s configuration, status, and control information. The transaction schedule allows flow control for some stream pipes. At the hardware level, this prevents buffers from underrun or overrun situations by using an acknowledgement (ACK) handshake to throttle the data rate. When acknowledged, a transaction may be retried when bus time is available. The flow control mechanism may permit the construction of flexible schedules that accommodate concurrent servicing of a heterogeneous mix of stream pipes. Thus, multiple stream pipes may be serviced at different intervals and with packets of different sizes.
FIG. 5: Time-out Limitations Of USB
FIG. 5 illustrates the data send and acknowledgement process in a USB system as related to transmission time-outs. Referring to FIG. 5, a Data Link 504 provides a transmission medium between a Source 510 such as a USB device and a Destination 520 such as a host computer system. At time T=0 Packet 502 may be sent from the Source 510 and received at the Destination 520 at time T=A. Then, as shown in FIG. 5, at time T=B ACK 504 may be sent from the Destination 520, and received by the Source 510 at time T=C. Now, as long as C (the time of the reception of the ACK 504 from Destination 520) is less than the time-out specification of the Source 510, the Source 510 may consider the transmission transaction a success and may proceed to send the next packet of data to the Destination 520. If the timeout is exceeded the failing packet may be retransmitted by the Source 510.
Limiting the amount of time for the turnaround acknowledgment may limit the usefulness of the data exchange process in that transmission cables may be strictly limited in length to avoid time-outs. Externally increasing this timing window may result in either the ability to add the time to do processing to the data exchange chain or time to increase of the time-of-flight in the medium of the signals themselves, thus increasing the maximum cable lengths allowed by the system.
FIG. 6A: A Technique For Solving The Time-Out Problem
One approach to circumventing the time-out limitations inherent in the USB protocol is to avoid one of the timing specifications, specifically the shorter of the two, which is the time-of-flight in the cable. This may be done by circumventing this aspect of the USB 1.0 process. Such an interface is illustrated by FIG. 6A. As FIG. 6A shows, a source local PC based Host Controller 108 may be connected through Link A 630 to a special interface 602 which fulfills all the PC""s USB needs for seeing less than one data bit in the cable at a single time in Link A 630. A special Hub 604 may be located at the device end of the system which provides the same service to the UBS Devices 110 attached to the system, that is, it assures that only one bit may be in the cable at the end at one time for Link C 650. It should be noted that Link A 630 and Link C 650 may be USB compliant.
In Link B 640, which is between the Interface 602 and the Hub 604, the xe2x80x9cone bit in the cable at one timexe2x80x9d rule of the USB specification is not applied. Only the overall packet response time issue is of concern. The length of the Link B 640 becomes a time/length accounting issue. As FIG. 6 shows, both the Interface 602 and Hub 604 represent process delays of 40 nS. This is also true for the end USB devices 110. Each of the standard cable lengths represents about 30 nS more delay. Given that there may only be a maximum of 415 ns in travel each way, and the cables and process each way take up roughly 160 nS (30+40+40+30 +20 (half of the last turn around)), approximately 255 nS remain for signal propagation through Link B 640. In a standard cable with 65% the velocity of the speed of light as a propagation rate, the maximum distance allowable for Link B is approximately 160 feet for the single run. The addition of the two 16 foot cables at each end may then permit the cable length to be expanded from about 15 feet to about 200 feet. In this way the USB rules may be violated to permit the extension of the line.
Disadvantages of the above solution include the fact that workable solutions typically involve non-standard solutions that can further exacerbate irregularities in the communications system, there may be a reduction of robustness and accuracy in the communications system, and finally, the above solution reduces the number of allowed Hubs to one from five and thus the number of USB Function Devices that the system can accommodate.
FIG. 6B: A Second Technique For Solving The Time-Out Problem
A second technique for addressing the time-out problem involves the placement of several USB hubs in series. Such a configuration is shown in FIG. 6B. As FIG. 6B shows, host computer 108 may be coupled through USB to a USB hub 611A. USB hubs 611B through 611E may be connected in series through USB. USB devices 110 may be connected to USB hub 611E. Using this technique, up to five hubs may be connected in series for a total distance of 30 meters (about 99 feet). This process, in effect, simulates additional in-line hubs allowing additional cable lengths corresponding to the process times (40 ns) of each hub. This techniques has, however, two basic problems:
1. By adding hubs in series to extend the distance between the computer and the final USB hub (assumed to be the xe2x80x9cwork placexe2x80x9d or xe2x80x9cdesk topxe2x80x9d), the total number of USB devices that may be connected to the system may be dramatically reduced. More specifically, as a USB hub is typically an eight port unit, and USB may typically handle up to 127 USB devices, the effective capacity of the USB may be reduced by a factor of 15 to accomplish the USB cable extension process. To retain full use of the USB system may still require a limit of 16 feet on cable lengths.
2. Even if the limitation on the number of USB devices is acceptable, the maximum distance that the USB hub may be extended is still less than 100 feet.
As mentioned above, it may be desirable to operate USB and associated USB peripherals at a remote location from the associated host computer. It may also be desirable to operate a display device, such as a video monitor, at a remote location. Therefore, improved methods for transmitting video signals in a remote USB system are needed.
A system and method is presented for operating a display device and one or more USB peripherals remotely from a host computer. The system may include a host computer system including a processor and a memory, a display device, one or more USB peripherals. The system may include a local extender operable to couple to the host computer system and a remote extender operable to couple to the local extender and to the display device and the one or more peripherals. The local extender may include a first video converter and a USBX host controller both of which may be operable to couple to the host computer system. The remote extender may include a second video converter operable to couple to the display device and a remote USB interface device operable to couple to the one or more peripherals. In one embodiment the USB interface device may be a Remote Root Hub.
The Remote Root Hub may be located a distance from the host computer system which is greater than a maximum distance specified in a USB protocol specification, e.g., greater than 5 meters. In one embodiment the distance may be greater than 10 meters.
In one embodiment the local extender may be comprised on the host computer system. In another embodiment the host computer system may be comprised on a card or xe2x80x9cbladexe2x80x9d which is installed in a chassis. In one embodiment, the local extender may simply be a component on the xe2x80x9cbladexe2x80x9d. Similarly, in one embodiment the remote extender may be comprised in the display device, wherein the one or more peripherals may be coupled to the remote extender via connections on the display device. In other embodiments the remote extender may be comprised on the keyboard, or any other peripheral device.
The local extender may be coupled to the remote extender through a non-USB compliant bus comprising four twisted wire pairs, which in one embodiment may comprise a category 5 cable. Three of the four twisted wire pairs may be operable to communicate Red, Green, and Blue (RGB) video signals from the local extender to the remote extender, and the remote extender may be operable to send the RGB video signals to the display device. One of the four twisted wire pairs may communicate peripheral data between the local extender and the remote extender, which may then send the peripheral data to, or receive the peripheral data from, the one or more USB peripherals.
The host computer system may generate a video signal intended for the display device, as well as one or more peripheral signals intended for the one or more peripherals. The first video converter of the local extender may receive the video signal, convert the signal to a form which is compatible with transmission over the cable, and transmit the signal to the second video converter in the remote extender. The USBX host controller may receive the one or more peripheral signals from the host computer system over an internal computer bus, such as a PCI bus, convert the signals to a non-USB compliant bus protocol, such as Universal Serial Bus Extension (USBX) packets, and transmit the USBX packets to the remote USB interface device in the remote extender. In addition, the remote USB interface device may receive on or more peripheral signals from peripheral devices, typically input devices such as a mouse or keyboard.
The video data may further comprise HSync and VSync video synchronization signals which may comprise pulses which may be converted from standard length pulses to short pulses by the local extender for transmission over the cable to the remote extender. The remote extender may further comprise a pulse sharper which may convert the short pulses back to standard length pulses for use by the display device. In one embodiment the HSync and VSync video synchronization signals may be transmitted over two of the three wire pairs used for transmitting the RGB video signals. In an alternate embodiment the HSync and VSync video synchronization signals may be transmitted over the one twisted wire pair used to communicate the peripheral signals to the remote extender. In this case the HSync and VSync video synchronization signals may be received by the Remote Root Hub which may send the signals to the pulse shaper, described above.
In one embodiment the remote extender may further include three delay components operable to couple to the three twisted wire pairs, wherein the three delay components may be operable to synchronize the Red, Green, and Blue video signals to compensate for different time-of-flight values for each of the three twisted wire pairs.
In one embodiment the remote extender may further comprise four equalization components coupled to the four twisted wire pairs, and which may compensate for attenuation of low (DC) and high frequency signals due to the length of the four twisted wire pairs.
Thus the system and method described above may allow the remote location of user interface peripheral devices such as keyboards and pointing devices (such as a mouse) from the host computer at a distance greater than that allowed by the USB specification. The system and method may also allow the remote location of a video monitor from the host computer using cabling used for extending the operational distance of USB peripheral devices. Thus, cabling requirements may be reduce from separate video and USB cables to a single thin category 5 cable which is a common and inexpensive component.